Developer documentation for testing setup

[robchallen]

Is your feature request related to a problem? Please describe.
Checking out the project and opening it in an IDE generates compile errors, preventing running of tests, unless the version of Java is updated to JDK17 in the pom.xml which is presumably undesirable. This also made deploying snapshots of forks on jitpack complex.

Describe the solution you'd like

• Instructions on how to set up maven toolchain correctly to compile and test.
• Ideally a solution that works on jitpack also

[robchallen]

N.B. The jitpack part of the issue could probably be solved with a jitpack.yml implementing a custom command:

https://docs.jitpack.io/building/#maven-projects

jdk:
  - openjdk9
before_install:
   - ./custom_setup.sh
install:
   - echo "Running a custom install command"
   - ./gradlew clean install -xtest
env:
   MYVAR: "custom environment variable"